CHICAGO — Albert Einstein said his biggest blunder was changing his equation describing space to add a term for repulsive energy. Such energy was needed, he thought, to keep the universe from collapsing.

But then in 1929 Edwin Hubble discovered that the universe is expanding. So Einstein renounced his repulsive energy. He shouldn’t have, though, because seven decades later astronomers found evidence that repulsive energy permeated space after all.